Method for producing a turbomachine part by means of a laser process

ABSTRACT

The invention relates to a method for producing a part by means of a laser beam, with a nozzle ( 1 ) that sprays a metal powder towards a substrate ( 5 ). Initially, the trajectory of the nozzle is defined in a pre-determined manner, and then, during the production of the part ( 7 ):
         a theoretical reference distance D 0  that has been previously recorded and a real distance which is then measured are compared, and
 
the trajectory of the nozzle is modified on the basis of a deviation threshold between said distances.

The present invention relates to a method for producing or repairing a turbomachine part by means of a laser beam. It also relates to a turbomachine part produced using this method.

In such a method, also known as

Laser Metal Depositio

, a nozzle sprays a metal powder towards a substrate so as to produce the part by means of successive depositions, one on top of the other, of layers or beads, in a deposition direction. The powder considered, which typically is a mixture of metal powders, is molten by the Laser beam. The thickness of each bead typically ranges from 0.05 mm to 1 mm. Low thickness should be preferred if the surface condition is to be optimized.

This method makes it possible to produce large dimension parts, specifically as regards height (Z axis in an orthonormal X, Y, Z system). However, it is difficult to obtain guaranteed final dimensions of the part (or portion of the part) thus produced.

Variations in the powder rate, the reloading speed of the spray nozzle, the laser power or the temperature of the part, may cause variations in dimensions, specifically variations in the height of the deposited layers. Locally, such variations may have little impact; but these may increase when and as the layers are deposited and even cause an unstable deposition resulting in the generation of a saw-teeth like deposition.

One aim here is to provide a solution to all or part of the drawbacks above.

For this purpose, the proposed method for producing or repairing by spraying metal powder using a laser beam is characterized in that:

-   -   initially, the trajectory of the nozzle is predetermined,     -   then, during the production of the part:         -   referring to an orientation parallel to said direction of             deposition of the layers, a theoretical reference distance             D0 that has been previously recorded and a real distance             which is then measured are compared, and         -   the trajectory of the nozzle is modified on the basis of a             non null deviation threshold between the theoretical             reference distance and the real distance measured.

Time-setting the initial production routine through one or more modification(s) in the initially defined trajectory of the nozzle will make it possible to be closer to the expected final dimensional characteristics of the part.

A better surface condition can thus be expected, without saw-teeth being as visible as before.

As it affects one or more geometrical parameter(s), this solution further avoids having to modify process parameters: laser power, reloading speed between two successive depositions of layers, powder rate, . . . .

In order to be able to guarantee that the height of the thus deposited material is the expected one, it is recommended that the regulation test implemented should:

-   -   for the definition of the predetermined trajectory of the nozzle         to include a definition of said trajectory along a Z axis, which         corresponds to said direction of deposition of the layers and a         height of the part,     -   for the real distance to be measured along said Z axis,     -   and for the trajectory of the nozzle to be modified along said Z         axis.

When implementing the above, it has been found that it could be more efficient not to modify the distance between the nozzle and the top of the layer opposite thereto, and rather, preferentially,

-   -   produce the part in successive steps, in said direction of         deposition of the layers, while taking the nozzle away from the         substrate (and thus, as from the second layer, from the         previously deposited layer),     -   and, —the predetermined trajectory of the nozzle including a         predetermined number of such steps, —and modify the trajectory         of the nozzle by changing said predetermined number of steps.

Similarly, it is provided:

-   -   that a predetermined number of said layers to be deposited         should correspond to the predetermined trajectory of the nozzle         (in the machine programme),     -   that a modified number of said layers still to be deposited         should correspond to the modified trajectory of the nozzle,     -   and that the trajectory of the nozzle should be modified by         substituting the modified number of layers still to be deposited         with said predetermined number of layers to be deposited.

Modifying, over time, the number of steps when successively depositing layers and/or the number of layers still to be deposited will a priori be easier and safer to be implemented and to be controlled than varying the thickness of such layers, during the production of the part.

In connection therewith, it is additionally proposed:

-   -   to produce the part by successively depositing layers having the         same thickness, by moving the nozzle away from the substrate,         for each layer, i.e. the direction of deposition of the layers,     -   and/or:         -   that a determined distance between the substrate and one end             of the nozzle opposite said substrate should correspond to             the theoretical reference distance,         -   and this determined distance should be maintained during the             production of the part, at the beginning or the end of the             deposition of each layer.

Keeping such

determined distance

and/or conditions of deposition aiming at depositing stacked layers having the same thickness will enhance the stability of the solution.

One advantage of the solution also relies in a quick production. Besides, it was found that waiting for some time was necessary for the uncertainties of accuracy in measurements to be low enough.

This is the context wherein it is provided that, during the production of the part, —said real distance should be measured,

-   -   and/or the trajectory modified,         only after several layers have been deposited, if said deviation         is reached.

Preferentially, it is recommended during the production of the part:

-   -   to carry out several successive measurements of said real         distance,     -   and, if said deviation is reached, —to modify the trajectory of         the nozzle only after several layers have been deposited, as         compared to the previous measurement.

Another problem to be solved was defining how to obtain the above-mentioned distance data.

A solution using a measuring autofocus camera has been preferred.

It has thus been proposed to use a camera with an autofocus system to obtain the theoretical reference distance and the real distance.

Another corollary problem to be solved was obtaining reliable measures, which are not dependent on the conditions of the creation of the molten bath on the part being produced, opposite, along the laser axis.

Specifically when the laser beam is emitted along said Z axis, measuring the theoretical reference and real distances away from said Z axis has thus been preferred, in parallel to this axis or at an angle (A) having a projection parallel to said Z axis.

It will thus be possible to prevent the camera from aiming at the molten bath.

The invention will be better understood and other details, characteristics and advantages of the invention will appear upon reading the following description given by way of a non-restrictive example while referring to the appended drawings wherein:

FIG. 1 schematically shows one end of the powder spray nozzle, in a selective powder melting machine of the

Laser Metal Deposition

type,

FIGS. 2, 3, 4 schematically show such a spray nozzle, which is then equipped with means for measuring the theoretical reference distance and the real distance in the course of production of the part, respectively at the beginning (FIG. 1) and then in the course of the production,

FIG. 5 is a block diagram of the control process of the invention, which aims at guaranteeing the reaching of (at least some of) the expected final dimensional characteristics of the part, and

FIG. 6 schematically shows a part obtained using this technique.

FIG. 1 shows a known nozzle 1 of a laser metal deposition machine 2. The nozzle sprays a mixture 3 of metal powders 3 a, 3 b onto a substrate 5 (FIGS. 2-4), so as to produce a part bearing reference 7 in FIG. 6.

The substrate 5 is a conventional support in the field, and adapted to successive layers 11 ₁, 11 ₂, 11 ₃, . . . , 11 i . . . of sprayed material being deposited thereon (FIGS. 3, 4), in connection with a laser beam 13 being emitted toward such substrate. The powder or mixture is thus molten to create a homogeneous and dense deposition onto the surface, which is molten too. Typically, such successive depositions or stacks are protected, all through the production, by a neutral gas, in order to prevent any oxidation problem. This technique makes it possible to execute wide depositions, of the order of 4 to 5 mm, and thinner depositions (500 μm wide). As the quick-production nozzle and the substrate are not in contact, no wear occurs.

Hereunder, we will disclose a situation where, as illustrated, the nozzle 1 sprays, in a layer deposition direction, here (substantially) vertically, along a Z axis, the metal power 3 towards the substrate, for making the height (portion) of the expected part.

The laser beam 13 is thus emitted toward the substrate, along the Z axis, and, in this case, the theoretical reference and real distances mentioned above will be measured in parallel to this axis or at an angle A (FIG. 4) having a projection 13 a parallel to the Z axis.

The method developed here could however be implemented along either one of the other two X, Y axes of the conventional orthonormal X, Y, Z system (FIG. 2). A horizontal aiming of the measuring camera 15 could thus be imagined along one of the X, Y axes.

In the preferred case shown: the nozzle 1 comprises two concentric cones 16 a, 16 b coaxial with the Z axis.

Originating from a laser source 17, and if need be, using a mirror 19, the laser beam 13 is emitted vertically towards the substrate 5 at the center of the central cone 16 a.

The mixture 3 of the metal powders 3 a, 3 b circulates in the external cone 16 b, and it is sprayed out of same, downwards, toward the substrate 5, via a carrier gas 21 b. Another gas 21 a surrounds the beam 13 in the internal cone 16 a.

The deposition of the material resulting from the mixture 3 may be uneven because of the laser beam 13. For instance, specifically if the two cones are no longer centered, more material may be spread on one side than on the other one.

Metal powders may be titanium alloys (TA6V, Ti71, 6242, . . . ), nickel and cobalt based alloys (Inco718, Hastelloy X, René77, René125, HA188) and steels (Z12CNDV12, 17-4PH).

The successive depositions of the layers 11 ₁, 11 ₂, 11 ₃ . . . 11 _(i) . . . of materials will thus be stacked on the substrate 5, until the expected part 7 is obtained.

The block diagram of FIG. 5 which is a synthesis of the main steps of production of such part, in one preferred embodiment of the invention, indicates that the nozzle and more generally the deposition machine 2 control program, has been created with the following successive steps:

-   -   step 27: initially, prior to starting producing the part,         measuring (in 27 a using the autofocus 45; see below) and         calculating (in 27 b) and storing into the memory 29 a         theoretical reference distance D0 between the surface of the         substrate 5 and one position 1 a of the nozzle, movable         therewith, here along the Z axis. A calibration with the initial         correlation of the camera autofocus system with the part to be         produced (or to be reloaded, in case of repair) will guarantee         precision and quality;     -   step 31: definition and storing into the memory 29 of a         predetermined trajectory of the nozzle 1, adapted to the         production of the part, to be followed by the nozzle, initially         and at least when starting the production of the part.

The following steps are then carried out successively in a sequence, during the production of the part:

-   -   step 33: whereas the nozzle has moved on its predetermined         trajectory, driven by the control program 34, measuring (in 33 a         using the autofocus 45) and calculation (in 33 b) and storing         into the memory 29 a real distance Di (i=1, 2 . . . n) between         the location 1 a of the nozzle and the free surface 35 i (i=1, 2         . . . n) of the layer 11 i, this layer (the last layer, if         several layers have been deposited onto the substrate, as         illustrated in FIG. 5),     -   step 37: comparison between the theoretical reference distance         D0 and the real distance Di, while referring to a predetermined         deviation threshold, between these two distances (D0−Di).

Two options then exist:

-   -   step 42: if the deviation threshold is reached (or exceeded),         the trajectory of the nozzle is modified (and recorded in the         memory 29), or     -   step 44: if the deviation threshold is not reached, the         predetermined trajectory of the nozzle is maintained.

In the mean time, a test step 39 or 41, respectively, has been carried out, with two options again:

-   -   either the manufacturing step is not the last one (i.e. the         deposition concerned is not that of the last layer 11 i) and the         aforementioned step 42 or 44 is then carried out,     -   or the deposition concerned is that of the last layer 11 i         (based on the initially set number or the modified number, in         the case of the preferred choice disclosed below), and the         manufacturing process then ends at the step 46 or 48, as         appropriate.

If step 42 or 44 has been reached, it means that at least one step of deposition is still to be carried out, and that a return, i.e. a loop is provided, in both cases, on one of the lines 50, again at step 33, to repeat the steps 33 to 39 or 41 a certain number of times, and so periodically reset the manufacturing program 34 if necessary, and adapt, in real time, the trajectory of the nozzle while measuring a real distance Di at each step 33.

As regards the modification in the trajectory at step 42, acting on the steps of forming the layers, and specifically on the number of layers still to be deposited has been preferred.

Specifically, it has been understood from the foregoing that the production of the part 7 is obtained by moving, in successive steps, the nozzle 1 away from the substrate 5, and when a layer has been deposited, from the previous layer 11 i of deposited material, when and as the layers are stacked.

The definition and the storing into the memory 29, in step 31, of the predetermined trajectory of the nozzle 1 will thus preferably include those of a predetermined number of such steps of depositions.

And it will be possible to modify the trajectory of the nozzle 1 by changing the predetermined number of steps.

In practice, it is recommended that the execution of one of the above layers 11 should correspond to one step of deposition.

Thus:

-   -   in step 31. when defining and storing into the memory 29 such         predetermined trajectory of the nozzle 1 into the linked driving         program 34, a predetermined number of layers 11 ₁, . . . 11 _(i)         to be deposited will then correspond thereto.     -   then, if step 42 is reached, a modified number of such layers         still to be deposited will correspond to the modified trajectory         of the nozzle in the program.

In this case, the trajectory of the nozzle will then be modified in the memory 29 by substituting said predetermined number of layers to be deposited with the modified number nc of layers 11 ₁, . . . 11 _(i) still to be deposited, with nc being a positive or negative integer.

In this respect, the above-mentioned threshold (D0−Di) leading to the step 39 or 41 will advantageously be equal to the thickness of a layer 11 i, i.e. typically 0.1 mm.

Then, if the distance Di is shorter than D0 by more than 0.1 mm, for example 0.2 mm, the program will add two steps of deposition, i.e. two layers. But it will remove three layers if the reading and the calculation indicate a distance Di of +0.3 mm as compared to D0. And there will be no modification in the steps if the reading and the calculation indicate a distance Di shorter or longer by less than 0.1 mm as compared to D0.

As mentioned above, in the illustrated example, this is according to the height of the part 7, (in particular) along the Z axis (or substantially parallel thereto):

-   -   that the predetermined trajectory of the nozzle 1 has been         initially specifically defined,     -   that the theoretical reference D0 and real D1, D2 . . . Di         distances are measured,     -   and that the trajectory of the nozzle is planned to be modified.

In practice, it is recommended that the/each “skip” mentioned above in the reloading program should be executed at the Z coordinate measured (D1 . . . Di) for X and Y values close to those where the nozzle 1 is located when its trajectory is reset. A restart routine can then also be used to manage the laser power and/or the reloading speed accordingly.

In such a situation of production controlled along the Z axis, it is also recommended that the expected completion of the part 7 should be performed by successive depositions of layers 35 a, . . . 35 i all having the same thickness e, on top of each other, by moving the nozzle away from the substrate, for each layer, here along the Z axis.

This will simplify the control of the correct progress in height of the part and will further avoid creating other surface irregularities (above-mentioned saw-teeth).

Preferably, this search for a relatively simple control of the compliance with the dimensional constraints of the part will also concern a limitation of the measures D1 . . . Di and/or the modifications in the trajectory of the nozzle.

Thus it is recommended that, during the production of the part, the real distance Di should be measured and the trajectory modified only after the deposition of several layers 11 i, if said deviation is reached.

In this respect, it may in particular be provided to perform several successive measurements of said real distance and (if said deviation is reached) to change the trajectory of the nozzle 1 only after the deposition of several layers with respect to the previous measurement.

In FIGS. 2-4 it can be noted that the schematic means for measuring the theoretical reference D0 and real D1 . . . Di distances comprise a camera (measuring camera 15) equipped with an autofocus system 45 (autofocus).

It is therefore preferably by using the autofocus that the initial distance D0 (when no layer has been deposited yet) as well as the real positions of the optical system 15 a of the lens of the camera 15 (zone 1 above), will be calculated and recorded, by aiming at the surface 35 i of the last deposited layer 11 i. Once the image is sharp thanks to the autofocus, the position of the nozzle relative to the part can thus be deduced.

The shootings will provide measurements parallel to the laser beam 13 directed towards the substrate, i.e. along the Z axis (or substantially parallel thereto).

Referring to the above explanations, the device will then operate as indicated hereunder, in connection with such distance measurements:

initially, as shown in FIG. 2 the lower free end 10 of the nozzle 1 (with the concentric cone tips 15 a, 15 b, being coaxial with the Z axis) is positioned at a so-called reloading distance Dc (which is thus the optimum distance between such end 10 and, initially, the free surface 5 a of the substrate, and then that 35i of the deposited layer 11 of material).

It should thus be understood that such reloading distance Dc will preferably be preserved at each step of the part production, with the nozzle moving away from the thickness e of one layer, for each deposited layer 11 i.

As a matter of fact, it is recommended that, if such determined distance Dc between the substrate and one end of the nozzle facing the substrate matches the theoretical reference distance D0, this distance Dc should be maintained during the production of the part, preferably when starting depositing each layer.

With the nozzle being set at the distance Dc, a clear image of the free surface 5 a of the substrate 5 taken by the camera at this initial time, using the autofocus 45, adjusted accordingly, will thus define the theoretical reference distance D0. The autofocus 45 is then preferably calibrated.

Then, the real distances D1, D2, . . . Di will then, as explained above, successively measured during the production of the part, with the nozzle still being a priori so positioned as to comply with the reloading distance Dc, using the autofocus 45 adjusted accordingly and as shown in FIG. 3.

FIG. 4 specifically shows the relative positions of the distance D0, D1, . . . Di measuring means (optical axis Z0 of the measuring autofocus camera 15) and of the nozzle 1, the center of which is gone through by the laser beam 13 (here the Z axis).

To prevent the camera from aiming at the molten bath, along the Z axis, which might make the focusing of the camera inaccurate, specifically when the surface receiving the laser beam is melting, the Z0 axis is here shifted aside (distance e1).

In connection therewith, two operating modes are possible:

-   -   either the optical axis is held vertical; refer to lens 15 a;         vertical axis Z0;     -   or the camera 150 (identical with the above-mentioned one 15) is         inclined at an angle A with a projection 13 a parallel to the Z         axis.

FIG. 6 also shows that the part 7 so produced could be one of the vanes of an annular row of substantially radial blades or vanes 47 of a disk 49 of an aircraft turbomachine which may be integral (one single part) with the disk. The blades 12 are connected at the radially internal ends thereof to one annular platform 51 which extends at the external periphery of the disk.

In addition to the fact that it could have been produced using the technique disclosed above, the vane 7 could also have been repaired, in case of wear. Reference 53 also refers to a section plane of such blade intended to be replaced. The free end surface of the segment 7 a of the blade still in position would define the above-mentioned surface 5 a of the substrate 5. 

1. A method for producing or repairing a turbomachine part by means of a laser beam, wherein a nozzle sprays a metal powder towards a substrate so as to produce the part by successive depositions of layers on top of each other, in one direction, therefore making the nozzle follow a trajectory, and wherein the trajectory of the nozzle is initially defined in a pre-determined manner, and then, during the production of the part: referring to an orientation parallel to the direction of deposition of the layers, a theoretical reference distance that has been previously recorded and a real distance which is then measured are compared; and the trajectory of the nozzle is modified on the basis of a non null deviation threshold between the theoretical reference distance and the measured real distance.
 2. The method of claim 1, wherein: defining the predetermined trajectory of the nozzle initially includes a definition of said trajectory along a Z axis corresponding to said direction of deposition of the layers and a height of the part, the real distance is measured along said Z axis, and the trajectory of the nozzle is modified along said Z axis.
 3. The method of claim 1, wherein: the part is produced while stepwise moving the nozzle away from the substrate, in said direction of deposition of the layers, the predetermined trajectory of the nozzle includes a predetermined number of such steps, and the trajectory of the nozzle is modified by changing said predetermined number of steps.
 4. The method of claim 1, wherein: a predetermined number of said layers to be deposited corresponds to the predetermined trajectory of the nozzle, a modified number of said layers still to be deposited corresponds to the modified trajectory of the nozzle, and the trajectory of the nozzle is modified by substituting the modified number of layers still to be deposited with said predetermined number of layers to be deposited.
 5. The method of claim 1, wherein the part is produced by successive depositions on top of each other of layers having the same thickness, by moving the nozzle away from the substrate, for each layer, in said direction of deposition of the layers.
 6. The method of claim 1, wherein during the production of the part, said real distance is measured and/or the trajectory modified only after the deposition of several layers, if said deviation is reached.
 7. The method of claim 1, wherein said deviation threshold between the theoretical reference distance and the measured real distance is equal to the thickness of a layer.
 8. The method of claim 1, wherein a camera with an autofocus system is used to obtain the theoretical reference distance and the real distance.
 9. The method of claim 2, wherein: the laser beam is emitted along said Z axis, and the theoretical reference and real distances are measured away from said Z axis, parallel to this axis or at an angle having a projection parallel to said Z axis.
 10. The method of claim 1, wherein a determined distance, between the substrate and one end of the nozzle facing said substrate corresponds to the theoretical reference distance, in said direction of deposition of the layers, and this determined distance is maintained during the production of the part, at the beginning or the end of the deposition of each layer. 